Optical systems comprise one or multiple (usually multiple) optical elements. The particular range(s) of wavelength(s) with which an optical system is used often dictates whether the optical system is refractive, reflective, or a combination of refractive and reflective. The term “optical element” can include, and is not limited to, any of various lenses, mirrors, reticles, windows, filters, and gratings, as would be used in an optical system or used alone.
Optical systems used for extremely demanding work must exhibit a very high level of optical performance. High optical performance requires that the optical elements be manufactured and placed relative to each other with extremely high accuracy and precision. These optical systems are also designed to exhibit minimal distortion and aberration, and frequently are configured to resist and/or compensate for adverse effects of shipping, storage, and prolonged service. For use in an optical system or alone, a “mounting” is a device or structure that essentially holds an optical element. Mountings for use in high-performance optical systems must be designed and constructed with extremely high accuracy and precision, must hold their respective optical elements without causing significant uncorrectable aberrations or other optical faults, and must not over-constrain their respective optical elements. One general class of mountings encompasses various clamps that engage the optical element itself or a tab, projection, flange, or other mounting structure integral with or attached to the optical element.
One application of high-performance optical systems as summarized above is microlithography, which generally is a technique used for establishing, by exposure printing, the locations and basic configurations of electronic elements on the substrates of semiconductor integrated circuits, displays, and other “microelectronic” devices. In these devices the number of individual circuit elements on a substrate can be extremely large (up to hundreds of millions). Hence, the individual elements are extremely small and in very close proximity to one another at high integration density. The trend is toward increased miniaturization of the elements and denser integration of the elements. Accommodating this trend requires microlithography systems providing increasingly finer resolution.
In microlithography the resolution of an optical system is affected by several variables including the wavelength of the energy used for making microlithographic exposures. To achieve progressively finer resolution of the exposures, increasingly shorter wavelengths of exposure light are being used or considered for use. Whereas most microlithography systems currently in use perform exposures using deep-UV light produced by excimer lasers (λ=248 or 193 nm), substantial effort is being directed to the development of a practical microlithography system utilizing extreme ultraviolet (EUV) light, having a wavelength in the vicinity of 13.5 nm, as the exposure light.
Most microlithography systems utilizing deep-UV exposure light have illumination-optical systems and imaging-optical systems that are at least partially refractive, in which the optical elements in the refractive optical systems are made of quartz and/or calcium fluorite. These systems include those utilizing certain of the excimer-laser wavelengths such as 193 and 248 nm. Use of EUV requires fully reflective optical systems.
Regardless of whether the optical system is refractive, reflective, or a combination thereof, it is critical that the optical elements of the system be manufactured with extremely high accuracy to achieve the necessary corrections of aberrations and to achieve the required levels of optical performance. In addition, these optical elements must be mounted in ways that withstand shipping and installation activity and that maintain the specified optical performance of the elements during use.
Hence, these and other high-performance optical systems include optical-element mountings that are configured not only to hold the optical elements securely but also to minimize or eliminate external effects such as gravitational sag, thermal stresses, and other deleterious variables acting on the optical elements. To such end, the mountings can be “kinematic” or “quasi-kinematic.”
In general, the mounting of an optical element or other object can be defined in terms of six independent coordinates or “degrees of freedom” (x, y, z, θx, θy, θz). Three of the coordinates (x, y, z) are translational along mutually perpendicular axes of an arbitrary coordinate system, and three are rotational (θx or “roll,” θy or “pitch,” and θz or “yaw”) about the respective axes denoted in the subscripts. In a true kinematic mounting, the number of constrained degrees of freedom (axes of free motion) and the number of physical constraints applied to the mounting total six. These physical constraints are independent and not redundant.
An example quasi-kinematic mounting is discussed in U.S. Pat. No. 6,239,924 to Watson et al., issued on May 29, 2001, and incorporated herein by reference. The mounting discussed in the '924 patent supports a lens on a set of mounting seats. Situated between the mounting seats are soft mounts that distribute the gravitational load of the lens. Another example is discussed in International Patent Publication No. WO 02/16993 A1, published on Feb. 28, 2002, in which the optical element is supported on bearing surfaces (seats) on a base member. The base member includes clamping members each including a pad member with a flexible thin-plate portion through which the optical element is clamped using bolts. An example kinematic mounting employing certain configurations of flexures is discussed in U.S. Pat. No. 6,922,293 to Watson et al., issued on Jul. 26, 2005, and incorporated herein by reference.
It is known to support an optical element, such as a deformable mirror, using multiple high-stiffness actuators such as PZT actuators as discussed, for example, in U.S. Pat. No. 5,037,184 to Ealey, issued on Aug. 6, 1991. Unfortunately, these types of mountings tend to over-constrain the optical element. Another deformable mirror-actuation system is disclosed in U.S. Pat. No. 6,989,922 to Phillips et al., issued on Jan. 24, 2006. Reference is also made to Hardy, “Active Optics: A New Technology for the Control of Light,” Trans. IEEE, Vol. 60, No. 6 (1978).
Optical members in high-performance optical systems typically are very sensitive to their environment. For example, optical elements can experience gravitational sag and/or significant thermal gradients, which can impair optical performance. Also, excessive holding force or holding force that is too constraining to the element may damage or deform the element or otherwise cause adverse optical effects. If the element is mounted by clamps (which ordinarily tend to concentrate holding forces), possible deformation of or damage to the element is usually a concern. To minimize adverse effects, it is generally desirable to reduce clamping force as much as possible while maintaining sufficient holding friction to support the optical element.
Deformation of an optical element by its mounting can be difficult to control or eliminate because deformation often comprises multiple components that are difficult to distinguish from one another and to characterize. For example, relatively large components of deformation many be caused by manufacturing variations or design shortcomings in the mounting and/or in the optical element itself. Relatively small and more localized components of deformation may arise from thermal effects on the optical element and/or its mounting. The components can exhibit static and/or time-varying components that are often complex. For example, each point at which the optical element is contacted by a mounting may result in translational and/or rotational stresses corresponding to one or more of the six degrees of freedom (6 DOFs) being applied to the element. The resulting complex strains and deformations may extend over substantial portions of the optical element from the points of contact.
Therefore, there is a need for improved mountings for optical elements, especially for use in high-performance optical systems.